In the sense of the present invention, a sound proofing assembly ensures <<insulation>> when it prevents the entry of acoustic waves at medium and high frequencies into the soundproof space, essentially by reflecting the waves towards the sources of noise or the outside of the soundproof space.
A sound proofing assembly operates by <<acoustic absorption>> (in the medium and high frequency domain) when the energy of the acoustic waves dissipate in an absorbent material.
A performing sound proofing assembly should operate both by ensuring good insulation and by absorption. In order to characterize the performance of such an assembly, the notion of sound reduction index NR is used which takes into account both notions of insulation and of absorption: this index may be calculated with the following equation:NR (dB)=TL−10 log(S/A)
wherein TL is the acoustic attenuation index (attenuation index hereafter) expressing insulation. The higher this index, better is the insulation.
A is the equivalent absorption surface area. The higher A, better is the absorption.
In order to achieve good sound proofing for example for an automobile passenger compartment, it is desirable to apply an assembly of materials which will give the possibility of suitably acting on both of these notions. This was described in many articles, in particular in the article <<Faurecia Acoustic Light-weight Concept>> by A Duval in 2002 during the SIA/CTTM 2002 conference in Le Mans.
In order to provide good acoustic insulation, the use of assemblies of the mass-spring type formed with a porous and elastic base layer is known, on which is placed an impervious layer with a heavy mass. This impervious layer of heavy mass generally has a high surface mass, in particular greater than 1 kg/m2. and an also high specific gravity of the order of 1,500 kg/m3 to 2,000 kg/m3.
Such acoustic assemblies provide good acoustic insulation, but are relatively heavy. Moreover their behavior is not very performing in absorption.
Moreover, in order to lower the mass of a sound proofing assembly, patent U.S. Pat. No. 6,145,617 describes an acoustic assembly of the <<bi-permeable>> type, in which the heavy mass layer is replaced with a porous layer. Such an assembly actually lightens the structure of the vehicle but especially proves to be effective in absorption and does not provide satisfactory insulation, as compared with a traditional mass-spring system.
Many attempts have been made for proposing a sound proofing assembly which is both sufficiently lightweight and sufficiently effective in insulation and absorption, i.e. providing a good attenuation index regardless of the frequencies.
However, the performances of the advanced solutions still remain insufficient, this essentially because of the insulation behavior which is lacking.